Simulating grass-legume dynamics: a phenomenological submodel

A simple recipe for modelling the dynamics of the legume component of a gra ss-legume pasture simulator is proposed, avoiding some of the difficulties associated with representing the pasture submodel as autonomous but interac ting grass and legume components. A target legume content of the sward is a ssumed to depend on the carbon: nitrogen ratio in the plant substrate carbo n and nitrogen pools (these represent the labile and easily mobilizable C a nd N pools in the plant). The rate at which the actual legume content appro aches the target content is proportional to the gross specific growth rate of the pasture. The canopy extinction coefficient and the dinitrogen fixati on rate for the pasture are adjusted according to the changing legume conte nt. The method has been incorporated into a generic single-species grasslan d simulator, the Hurley Pasture Model. Seasonal changes in legume fraction and associated variables are simulated. Next, the response of the legume fr action to step changes in nitrogen fertilizer application, carbon dioxide c oncentration, rainfall and temperature are predicted for a grazed pasture. Yield from frequent harvesting is also examined for four treatments: ambien t and elevated carbon dioxide x low and high nitrogen fertilizer applicatio n. Qualitatively, the simulations agree well with experimental findings. Th is indicates that some important aspects of grass-legume competition could operate primarily through the pasture carbon: nitrogen substrate ratio, Thi s ratio may determine those characteristics of morphology, growth and funct ion that largely define the differential success of the two components of g rass-legume swards; however, causation would not be proved (as far as this is scientifically possible) without a detailed mechanistic model, The appro ach may be useful for the investigation of management and climate-change pr oblems in grassland. (C) 2001 Annals of Botany Company.